The present invention relates generally to magnetic resonance (MR) imaging and, more particularly, to a method and system of determining in-plane motion of a subject from which MR data is acquired in a Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction (PROPELLER) acquisition, or variant thereof.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization”, MZ, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx, Gy, and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Fast Spin Echo (FSE) imaging is an imaging technique commonly used as an efficient method of collecting MRI data with minimal artifact. Generally, FSE requires that the refocusing B1 pulses be applied between each echo such that their phase is substantially identical to that of the initial spin phase after excitation, commonly referred to as the “CPMG” condition. If this condition is not met, the resulting MR signal is generally highly sensitive to the strength of B1, and therefore will generally decay rapidly in successive echoes.
FSE imaging is an imaging technique that has been implemented with a number of pulse sequence designs. For example, PROPELLER is an FSE technique that encodes an MR signal by collecting data during an echo train such that a rectangular strip, or “blade”, through the center of k-space is measured. This strip is incrementally rotated in k-space about the origin in subsequent echo trains, thereby allowing adequate measurement of the necessary regions of k-space for a desired resolution. PROPELLER is particularly effective at reducing the effects of patient motion during data acquisition. Accordingly, PROPELLER is particularly useful for imaging patients, such as children, who tend to move or tremor during data acquisition.
In conventional PROPELLER scans, redundant low-frequency k-space data from overlapping blades is compared to one another to determine in-plane motion of the patient between acquisition of the k-space blades. In this regard, the low-frequency k-space data is “gridded” from a Cartesian lattice to a polar lattice to estimate the patient's in-plane motion relative to a k-space reference blade or image. As such, in-plane motion in a given k-space blade is estimated by computing the convolution of the given k-space blade with a k-space reference blade. Nevertheless, while reasonably effective, the convolution is extremely smooth which makes identification of the maximum point in the convolution difficult to identify. The maximum point corresponds to the patient's position during acquisition of the given k-space blade relative to the k-space reference blade or image and is used to determine appropriate parameters of a motion correction algorithm. Accordingly, there is a need to make the maximum point of the convolution more conspicuous and, thus, easier to identify.
It would therefore be desirable to have a system and method for evaluating MR data, acquired with PROPELLER or a variant thereof, to determine in-plane motion of a subject during data acquisition that more significantly identifies a maximum peak of the convolution of a k-space blade with a k-space reference blade.